So the windows are seated on the external skin so we have a big internal reveal of about 250mm. It has eps beads in the cavity. the diathonite will line the internal reveal over the cavity with a mesh to a depth of about 25mm.

So it's the cill on the inside you are concerned about? (technically called the 'window board', I think)

The window board sits on top of whatever is beneath it (diathonite plaster in your case) and usually extends sideways past the window edge a bit. The plaster over the sides of the reveal will cover the edge of the window board, so you need to think about sequencing. Window boards are often made of MDF, or timber. MDF has the benefit of stability in the highly variable environment a window board encounters, but timber can be more attractive.

IIRC we didn't fix our window boards in place, just held them in place with the plaster, because I was worried about holes in the waterproof & airtight membrane underneath it in our case.

What I mean is what is the insulation layer along the horizontal bit, rather than the board itself. Although it seems like you are suggesting why not just use diathonite everywhere? My only thought is if you use an insulating board or insulating plaster, then doesnt the moment you put wood over the top to finish and touch the frame become a thermal bridge of sorts, albeit that wood is not massively conductive? Just a lot more conductive than the insulation material it sits on?

Posted By: delpradoAlthough it seems like you are suggesting why not just use diathonite everywhere?

I'm not suggesting it - I thought you were telling us that was what you were going to do.

My only thought is if you use an insulating board or insulating plaster, then doesnt the moment you put wood over the top to finish and touch the frame become a thermal bridge of sorts, albeit that wood is not massively conductive? Just a lot more conductive than the insulation material it sits on?

Yes, but it's pretty minor. The inside surface of the window is supposed to be warm after all. If you're really concerned I suppose you could have a small gap filled with e.g. a mm or two of EPS and sealed with silicone on top. But that seems like way too much effort for too small a reward to me.

I fitted 100mm wood fibre internal insulation in a bedroom and used 10mm of aerogel insulation around the window reveal. This was then covered with lime plaster and then a riven slate window board was fitted. The 100mm insulation added to the natural thickness of our stone walls, giving us a total thickness of 500mm. I originally considered an oak window board but was concerned about such a wide board warping, so fitted slate instead.

The slate is surrounded on all sides by the aerogel insulation so there are no cold bridges. It looks fantastic too as the black slate matches the black powder coated window frames above it and the cast iron radiator beneath it.

The windows should really be seated in line with the insulation in the cavity.

Posted By: delpradoMy only thought is if you use an insulating board or insulating plaster, then doesnt the moment you put wood over the top to finish and touch the frame become a thermal bridge of sorts,